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Ain Shams University

Faculty of Engineering

Mechanical department

2nd year Mechanical

Lathes

By/Ahmed Mohamed ahmed Mohamed

&

Saad Sayed Saad

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Lathes and its operation

Lathes are common and the oldest machine used in industry which are

employed for turning, threading, cutting, drilling, boring and wide ranges of

other operation.

Table (1-1) shows list of operations can be made on lathe machine and their

relevant tool.

Metal cutting lathes may differ in size and construction. Among these are

the general-purpose machines that include universal engine lathes, plain

turning lathes, facing lathes, and vertical turning and boring mills.

Turning operation

turning operation and other operations that can be made on lathe m/c , the

primary cutting motion (rotary) is imported to WP, and the feed motion (s)

(in most cases straight along the axis of the WP) is imparted to a single-

point tool. The tool feed rate (s) is usually very much smaller than the

surface speed (v) of the WP. The figure 1 shows the main parameters in

turning including that:

Figure 1

1- Cutting speed

=

1000( )

Where

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D =initial diameter of the WP (mm)

n =rotational speed of the WP (rpm)

2. Rotational speed n

3. Feed rate s , which is the movement of the tool cutting edge in

millimeters per revolution of the WP (mm/rev).

4. Depth of cut a, which is measured in a direction perpendicular to the

WP axis, for one turning pass.

a = D d/2 (mm)

where

d is the diameter of the machined surface.

5. Undeformed chip cross-section area Ac

= . = . (2)

where

h = chip thickness in millimeters (h = s.sin mm)

b =contact length in millimeters

=cutting edge angle (setting angle)

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construction of the engine lathe:-

Figure 2

engine lathes are widely employed in job and lot production, as well as for repair

work. Parts of very complex forms may be machined by this lathe. Its size varies from

small bench lathes to heavy-duty lathes for machining parts weighing many tons.

(Figure 2) illustrates a typical engine lathe with its various parts. The bed carries the

headstock, which contains the speed gearbox.

The bed also mounts the tailstock whose spindle usually carries the dead center. The

work may be held between centers, clamped in a chuck, or held in a fixture mounted

on a faceplate. If a long shaft is to be machined, it will be insufficient to clamp one

end in a chuck; therefore, it is necessary to support the other end by the tailstock

center (can be moved by handwheel). In many cases when the length of the shaft

exceeds 10 times its diameter ( > 10 D), a steady rest or follower rest is used to

support these long shafts (various ways of holding WP will be explained later).

Single-point tools are clamped in a square turret/tool post (hold 4 cutting tools, which

can be easily indexed for use as required as) mounted on the carriage. Tools such as

drills, core drills, and reamers are inserted in the tailstock spindle after removing the

center. The carriage, to which the apron is secured, may traverse along the guide ways

either manually or powered. The cross slide can also be either manually or power

traversed in the cross direction.

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Surfaces of revolution are turned by longitudinal & transversed feed of the carriage.

The cross slide feeds the tool in the cross direction to perform facing, recessing,

forming, and knurling operations.

Power traverse of the carriage or cross slide is obtained through the feed mechanism.

Rotation is transmitted from the spindle through change gears and the quick change

feed gearbox to either the lead screw or feed rod. From either of these, motion is

transmitted to the carriage.

Powered motion of the lead screw is used only for cutting threads using a threading

tool. In all other cases, the carriage is traversed by hand or powered from the feed rod.

Carriage feed is obtained by a pinion and rack fastened to the bed. The pinion may be

actuated manually or powered from the

feed rod. The cross slide is powered by

the feed rod through a gearing system in

the apron. (Figure 3.3 shows isometric

view for mechanism of lathe arpon) .

During thread cutting, the half nuts (9)

are closed by the lever(10) over the lead

screw(1) .

Figure 3

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Holding the workpiece on lathe engine:-

WP fixation on an engine lathe depends mainly upon the geometrical features of the

WP and the precision required. The WP can be held by:-

1. Between centers : A dog plate(1) and a lathe dog (2) are used (Figure 4). It is an

accurate method for clamping a long WP. The tailstock center may be a dead center

(Figure 5), or a live center (Figure 6), when the work is rotating at high speed. In

such a case, rests are used to support long WPs to prevent their deflection under the

action of the cutting forces. The steady rest (Figure 7) is mounted on the guideways

of the bed while the follower rest (Figure 8) is mounted on the saddle of the

carriage.

Figure 4

Figure 5

Figure 6

Figure 7 Figure 8

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2. Mandrel : Mandrels are used to hold WPs with previously machined holes. The

WP to be machined (2) is tightly

fitted on a conical mandrel, tapered

at 0.001, and provided with center

holes to be clamped between centers

using a dog plate and a lathe dog

(Figure 9). The expanding mandrel

(Figure 10) consists of a conical rod (1), a split sleeve (2), and nuts (3 and 4).

The work is held by expansion of a sleeve (2), as the latter is displaced along

the conical rod (1) by nut (3). Nut (4) removes the work from the mandrel.

There is a flat (5) on the left of the conical rod used for the setscrew of the

driving lathe dog.

Figure 10

3. Chuck : The most commonly employed method of holding short work

is to clamp it in a chuck (Figure 11). If the work length is

considerably large relative to its diameter, supporting the free end

with the tailstock dead or live center (Figure 12) is also used. Chucks

may be universal (self-centering) of three jaws, which are expanded

and drawn simultaneously (Figure 13); or they may be independent of

four jaws (Figure 14). The three-jaw chucks are used to clamp

circular and hexagonal rods, whereas the independent four-jaw chucks

are especially useful in clamping irregular and nonsymmetrical WPs.

Air-operated (pneumatic) chucks are commonly used in batch or mass

production by increasing the degree of automation (Figure 15). The

piston (1) is attached to a rod that moves it to the right or to the left

depending on which chamber of the pneumatic cylinder is fed with

compressed air. The end of the rod is connected to three levers (2),

which expand jaws (3) in a radial direction to clamp or release the

WP.

Figure 9

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Figure 12

Figure 13

4. faceplate : Large WPs cannot be clamped in a chuck and are, therefore, mounted

either directly on a faceplate (Figure 16), or mounted on a plate fixture (2) that

is attached to faceplate (1) (Figure 17). The work (3) and angle plate (2) must be

counterbalanced by using the counterweight (4) mounted at the opposite

position on the faceplate. The plate fixture has been proved to be highly

efficient in machining asymmetrical work of complex and irregular shape.

Figure 11

Figure 14

Figure 15

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How to set engine lathe for taper turning ?

Taper turning are turned by employing one of the four following methods:-

1. swiveling the compound rest to the required angle : Before performing the

operation, the compound rest is to be clamped in this position. The tool is fed

manually by rotating handle. This method is used for turning short internal and

external tapers with large taper angles, while the work is commonly held in a

chuck and a straight turning tool is used (Figure 18).

2. using a straight-edge broad-nose tool : The tool of width that exceeds the taper

being turned is cross-fed. The work is held in a chuck or clamped on a faceplate

(Figure 19).

3. setting over the tailstock: The angle of taper should not exceed 8. Since the

turned surface is parallel to the spindle axis, the powered feed of the carriage

can be used while the work is to be mounted between centers as shown in

(Figure 20). Before turning cylindrical surfaces, it is a good practice to check

whether the tailstock is not previously set over for taper turning; otherwise,

tapered surfaces are produced.

4. using a taper-turning attachment : This is best suited for long tapered work. The

cross slide is disengaged from the cross feed screw and is linked through the tie

to the slide (Figure 21).

Figure 16 Figure 17

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Figure 18

Figure 19

Figure 20

Figure 21

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Capstan & turret lathes: The distinguishing feature of this type of lathe is that the tailstock of an

engine lathe is replaced by a hexagonal turret, on the face of which multiple

tools may be fitted and fed into the work in proper sequence. Due to this

arrangement, several different types of operations can be done on a job without

re-setting of work or tools, and a number of identical parts can be produced in

the minimum time. The dimensional control is effected by means of

longitudinal (for lengths) and traversal (for diameters) adjustable stops.

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No Center lathe Capstan / turret lathe

1 It is a manually operated lathe It is a semiautomatic lathe

2 It has only one tool post tool

changing time is more

Front and rear tool posts are available.

Tool changing time is less

3 It has tail stock It has turret head instead of tail stock

4 Only one tool can be fitted in the tail

stock

Six different tools can be fitted in the

turret head.

5 Number of speeds is less Number of speeds is more

6 Tool changing time is more Tool changing time is less

7 The machine should be stopped for

changing tool

Tool can be changed without stopping

the machine

8 It is not suitable for mass production It is suitable for mass production

9 No feed stops to control the tool The tools are controlled by feed stops

10 The tool is centered manually after

changing the tool

The tool is centered automatically

11 Only one operation is done at a time More than one operation can be done at

a time

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No Capstan lathe Turret lathe

1 It is a light duty machine It is a heavy duty machine

2 The turret head is mounted on the ram

and the ram is mounted on the saddle

The turret head is directly mounted on

the saddle and the saddle slides over

the bed ways

3 The saddle will not be moved during

machining

The saddle is moved along with the

turret head during machining

4 The lengthwise movement of turret is

less

The lengthwise movement of turret is

more

5 Short work pieces only can be

machined.

Long work pieces can be machined

6 It is easy to move the turret head as it

slides over the ram

It is difficult to move the turret head

along with saddle

7 The turret head cannot be moved

crosswise

The turret head can be moved

crosswise in some turret lathes

8 As the construction of lathe is not

rigid, heavy cut cannot be given

As the construction of lathe is rigid,

heavy cut can be given

9 It is used for machining work pieces up

to 60mm diameter

It is used for machining work pieces

up to 200mm diameter

10 Collet is used to hold the work piece Jaw chuck is used to hold the work

piece

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CAPSTAN AND TURRET LATHE MECHANISM:

1. Bar feeding mechanism:

After a job is complete and part off, the collet is opened by moving the lever manually

rightward to withdraw the push force on the collet. Further moving of the lever in the

same direction causes forward push of the bar with the help of the ratchet paul system

shown. After the projection of the bar from the collet face to the desired length

controlled by a pre-set stop stock generally held in one face of the turret or in a separate

swing stop, the lever is moved leftward resulting closing of the collet by clamping of

the barstock. Just before clamping of the collet, the leftward movement of the lever

pushes the bar feeder (ratchet) back freely against the paul.

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2. Turret indexing mechanism:

The turret (1) is mounted on the spindle (5), which rests on a bearing on the turret

saddle .The index plate (2), the bevel gear (3) and the indexing ratchet (4) are keyed to

the spindle (5). The plunger (14) fitted within the housing and mounted on the saddle

locks the index plate by spring pressure (15) and prevents any rotary movement of the

turret as the tool feeds into the part. A pin (13) fitted on the plunger (14) projects out

of the housing. An actuating cam (10) and the indexing pawl (7) are attached to the

lathe bed (9) at the required position. Both the cam and pawl are spring loaded. As the

turret reaches the backward position, the actuating cam (10) lifts the plunger (14) out

of the groove in the index plate due to the riding of the pin (13) on the bevelled surface

of the cam (10) unlocking the index plate (2). The spring loaded pawl (7), which by this

time engages with a groove of the ratchet plate (4), causes the ratchet to rotate as the

turret head moves backward. When the index plate or the turret rotates by one sixth

of a revolution, the pin (13) and the plunger (14) drops out of the cam (10) and the

plunger locks the index plate in the next groove. The turret is thus indexed by one sixth

of the revolution and again locked in the new position automatically. The turret

holding the next tool is now fed forward and the pawl is released from the ratchet

plate by the spring pressure. The ratio of the teeth between the pinion and gear are so

chosen that when the tool mounted on the face of the turret is indexed to bring it to

the cutting position, the particular stop rod for controlling the longitudinal travel of

the tool is aligned with stop (12). The setting of the stop rods (8) for limiting the feed

of each operation may be adjusted by unscrewing the lock nuts and rotating the stop

rods on the plate. Thus six stop rods may be adjusted for controlling the longitudinal

travel of the tools mounted on the six faces of the turret.